The invention relates to a frame-proofed polymer composition.
Elastic elements, made for example from rubber, having no flame-retarding or fire-retarding properties per se, are required for some applications, partly on the basis of standards or statutory requirements. Previously, natural rubber or chloroprene rubber (CR) has been used as base polymers for dynamic applications. They have excellent mechanical properties and wear characteristics, but no flame-retarding or fire-retarding properties that meet the requirements of EN 45545-2. This relates in particular to problems concerning smoke density (NR, CR) and toxicity (CR).
Achieving low flammability, in particular according to standard EN 45545-2, with the associated high requirements in respect of flame propagation, optical smoke density, smoke toxicity and heat release rate, requires special attention to the selection of the polymers. The high requirements regarding smoke density and toxicity essentially preclude, for example, the application of halogen-containing polymers, e.g. chloroprene rubber (CR) or R-group elastomers with unsaturated main chains, e.g. nitrile rubber, for example HNBR. M-group olefin polymers having a low resistance to crack formation and growth under static and dynamic stress, and poor resistance to lubricating oils and greases, such as vinyl-acetate-containing thermoplastic polymers, are preferably used, in order to meet the requirements with respect to smoke gas density in toxicity.
It is also known to use elastomers with large amounts of flame retardants and/or fire retardants, the material properties however being strongly affected by the incorporation of such substances, wherein an element made from such rubber can no longer fulfil the static and dynamic properties required for the application.
In a dynamic application, for example as a spring element or damping element or similar, typically hydrodynamically loaded elements, on vehicles for example, cannot on the other hand satisfactorily meet the required fire protection provisions using known elastic elements based on NR and CR.
Currently, for example for elastomer articles in the area of rail vehicles, the problem arises that both EN 45545-2 HL3 (e.g. R9, R22/23) as well as the requirements of DIN 5514 (profile) and/or of BN 918043 (elastomers for technical purposes) are not be equally satisfied using a single material.
In order to solve this problem, attempts have been made to provide material combinations in order to keep the protected elastic core free from flame retardants. Such composite elements are described for example in DE 38 31 894 A1 or WO 2010/069842.
Further polymers, flame-retardant-containing mixtures from ethylene vinyl acetate with ethylene propylene diene monomer rubber are known from the prior art. In most cases, cross-linking is achieved by peroxide cross-linking or by irradiation. Such mixtures are primarily used for coating cables or electrical lines. Thus, for example, flame-proofed compositions of EVA, EPDM and LLDPE are described in EP 2 343 334 A2, which comprise a peroxide cross-linking system formed through dicumyl peroxide. These materials are mostly based on EVA with small additions of EPDM/EPM or PE.
Numerous polymer mixtures with dynamically cross-linked thermoplastic elastomers (TPV and/or TPE-V) are also known. These are two-phase systems, in which finely dispersed cross-linked rubber particles are incorporated in a continuous plastic matrix. At room temperature, thermoplastic elastomers behave comparably to conventional elastomers, but are however plastically deformable on application of heat and exhibit thermoplastic behaviour at elevated temperatures. This is undesirable for use as damping components, for example for rail vehicles, where the components need to retain their elastic properties at higher temperatures. Thermoplastics elastomers are accordingly unsuitable for this purpose.
Further sulfur cross-linked mixtures of EPDM and EVA are described in WO 2014/019008, which can be filled with very large amounts of flame retardants and have accordingly good flame resistance, but which nevertheless retain their advantageous mechanical properties due to a special sulfur cross-linking system.
It is also often sought to use plasticisers in order to correct the mechanical properties impaired by the higher proportion of flame retardants, in particular in terms of hardness and stiffness. Such elastomer mixtures, for example those based on EPDM or EPM, typically contain high proportions of mineral oil plasticisers in order, in particular, to improve the processing properties (flowability) and to adjust the mechanical characteristics such as the hardness and modulus. Typical quantities used are from 50 to 200 phr (parts per hundred rubber) with respect to the polymer.
Mineral oils are petroleum products and complex compositions of hydrocarbon mixtures and comprise paraffinic, naphthenic and aromatic components. For non-polar or weakly-polar olefin elastomers, mineral oils are used for reasons of polarity and thus compatibility, which should have a proportion of aromatically bonded carbon atoms (Caromatic) not greater than 50 wt. % with respect to 100 wt. % mineral oil. For reasons of polarity, only mineral oils with predominantly paraffinic or naphthenic components can be used for these constituents.
A typical formulation of such a known polymer composition comprises the following composition:
EPDM 100 phr
carbon black 20 phr
aluminium hydroxide (ATH) 180 phr
zinc borate 50 phr
plasticiser 45 phr
cross-linking chemicals: 12 phr
processing aids: 5 phr
The content of water-releasing flame retardants in this case is 230 phr, Such mixtures can fulfil the fire protection requirements of EN 45545-2 (R22/23, R9), but are however disadvantageous due to the poor mechanical properties resulting from the high content of flame retardants:
hardness: 64 Shore A
density: 1.47 g/cm3 
elasticity: 31%
tensile strength 4.2 MPa
elongation at break 560%
wear volume: >600 cm3 